infra: Send 50% of mac_chromium_rel_ng traffic to LUCI.

R=jchinlee@chromium.org

Bug: 731543
Change-Id: I84441f7d1a4b3b3a1ff620518019b4b93ab88b2b
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/1006265Reviewed-by: Jao-ke Chin-Lee <jchinlee@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549689}

infra/config/branch/cq.cfg

@@ -161,7 +161,7 @@ verifiers {
       }
       builders {
         name: "mac_chromium_rel_ng"
-        equivalent_to { bucket: "luci.chromium.try" percentage: 0 }
+        equivalent_to { bucket: "luci.chromium.try" percentage: 50 }
       }
     }
     buckets {